The long-term goal of our work is to understand how heparanase promotes the aggressive behavior of tumors and to use that knowledge to develop curative cancer therapies. We have identified multiple mechanisms through which heparanase drives cancer progression, metastasis and chemoresistance and established heparanase as a viable target for cancer therapy. Moreover, we catalyzed collaborative efforts aimed at developing and testing novel anti-heparanase drugs and demonstrated their efficacy in pre-clinical models of cancer leading to a currently ongoing clinical trial. The challenge now is to maximize anti-heparanase therapy to enhance its anti-tumor effects; a goal that will be attained through a thorough understanding of heparanase mechanisms of action. The overarching hypothesis guiding our work is that heparanase is a master regulator of the aggressive phenotype of cancer, an important contributor to the poor outcome of many cancer patients and a prime target for therapy. This hypothesis is supported by studies focused predominantly on the impact of heparanase expressed by tumor cells. Our latest discoveries have revealed two novel, previously unexplored mechanisms related to heparanase regulation of cancer. We found that: (i) heparanase expressed by non-tumor (host) cells within the microenvironment can substantially contribute to tumor progression, and (ii) tumor secreted exosomes can shuttle heparanase to recipient cells and enhance their chemoresistance. Our working hypothesis for Aim 1 is that heparanase produced by host cells acts within the tumor microenvironment to support and accelerate tumor growth, metastasis and chemoresistance. This will be studied in pancreatic carcinoma and myeloma tumors growing in mice that express different levels of host heparanase (transgenic and knockout mice). We will investigate the type of host cells involved (e.g., macrophages, stromal cells), their impact on tumor progression and chemoresistance and the ability of heparanase inhibitors to block those effects of host heparanase. Our working hypothesis for Aim 2 is that heparanase present as cargo in exosomes is delivered to recipient tumor cells and enhances their chemoresistance. We will determine mechanistically how exosomes drive chemoresistance and investigate the potential of anti-heparanase therapy for inhibiting exosome-mediated chemoresistance. In addition, work in Aim 3 will develop new, highly specific anti-heparanase monoclonal antibodies as anti-cancer therapeutics. The proposed work is significant and innovative because it couples the discovery of new heparanase mechanisms of action with the objective of maximizing anti-heparanase therapy and improving patient outcome.